1. Field of the Invention
This invention relates to biological methods and products useful in agriculture. More specifically, the present invention is directed to a method for controlling diseases caused by Fusarium moniliforme, biological control agents useful in such a method, and biological agents useful in agriculture.
2. Description of the Prior Art
Fungi of the genus Fusarium are responsible for numerous crop plant diseases around the world. These fungi are particularly harmful to cereal crops, such as corn, sorghum, and wheat. For example, one or more species of Fusarium can be isolated in every season from nearly every vegetative or reproductive part of corn in every corn-growing region of the United States. A particular species of Fusarium can occur singly or as part of a complex of Fusarium species or fungi or other genera. Depending upon weather, soil and the particular cereal variety, damage to the crop can range from negligible to extensive. A common occurrence, for example, is for a Fusarium diseases caused by Fusarium fungi include seedling blight, root rot, stalk rot, and ear rot.
Fusarium moniliforme is the most widely prevalent and economically important Fusarium found on corn in the United States, and is the most frequently isolated Fusarium species in shelled corn. Seed lots with 100% infection are not uncommon. It is also the most frequent cause of ear rot, and has been implicated in seedling blight and root rot. One of the primary diseases affecting the corn crop in the United States is stalk rot, and Fusarium moniliforme is considered to be the predominant stalk rot fungus in at least 10 states: Florida, Idaho, Iowa, Minnesota, Nebraska, New Jersey, North Carolina, Pennsylvania, South Carolina, and Virginia. F. moniliforme is also known to produce a phytotoxin and at least five chemically distinct classes of mycotoxins.
Fusarium graminearum is found less often in corn kernels than F. moniliforme, primarily occurring in the humid sections of the corn belt east of the Mississippi River and along the Atlantic seaboard. F. graminearum is a cause of ear rot and seedling blight, and one of the important causes of stalk rot in the New England, Mid-Atlantic and Northern corn belt states. F. graminearum also produces mycotoxins in kernels and stalks.
Additional species of Fusarium fungi that are of lesser importance than F. moniliforme and F. graminearum in corn disease include F. graminum, F. tricinctum, F. oxysporum, and F. soloni. While these species are mainly of importance as parts of disease complexes, F. tricinctum has been reported to produce mycotoxins in kernels and stalks.
Most Fusarium fungi are not believed to be strong pathogens. Generally, they produce major symptoms and damage crops only when the crops are under stress. The advance of the fungi in the plant is determined largely by what stress is occurring and the rate of cell death in plant tissues. Fusarium fungi have a wide host range of cereals and grasses and very effective methods of survival in soil or plant residues. Wherever grasses or cereals are grown in the United States, Fusarium infection of seedlings, roots, stalks, or ears is always possible. Thus, a method of controlling Fusarium infection in cereal crops is highly desirable. See generally, Fusarium Diseases, Biology and Taxonomy (P. E. Nelson, T. A. Toussoun, and R. J. Cook, eds. 1981); Christensen and Wilcoxson (1966) Amer. Phytopathol. Soc. Monogr. No. 3, 59 p.; Koehler (1959) Illinois Agric. Exp. Stn. Bull. 658. 90 p.
Attempts have been made to control fungal infections of plants by biological means. For example, Kommedahl and Mew (Phytopathology, Vol. 65, 2396-300, 1975) have reported that three corn hybrids were coated with Bacillus subtilis and Chaetomium globosum to determine the effect on seedlings, final stands, stalk rot, and yields in the field, in comparison with the standard chemical seed treatment, captan. Stalk rot and breakage were less with the organism- and captan-coated seeds compared to non-coated seeds. Grain yields, however, were generally higher for the captan-coated seeds than for the organism-coated seeds. It was concluded that the organisms were not as consistent as captan in protecting the plants, and that additional research was required before the use of organisms for coating corn kernels would be commercially feasible. Kommedahl and Mew, supra, also reported that captan is widely used as a seed treatment because it is reliable, easy to apply, and inexpensive. It was also reported, however, that captan is not always a good seed treatment under all conditions. In particular, Kommedahl and Mew, supra, reported that under prolonged conditions of low soil temperature and high soil moisture, biological controls proved superior to captan in reducing root infections. This was attributed to the possible multiplication of organisms and their growth from the seed to the root surface. For additional discussion of biological control of fungal infections in plants see generally: R. J. Cook, BIOLOGICAL CONTROL OF PLANT PATHOGENS (Amer. Phytopathol. Soc. 1982); Burges, H. D. (ed.) (1981) Microbial Control of Pests and Plant Disease 1970-1980, Academic Press, New York; Baker, Phytopathology Vol. 58, 1395-1401; Kommedahl and Chang, (1966), Phytopathology Vol. 56, 885; Kommedahl, (1972) Plant Dis. Rep. Vol. 56, 861-863; Mitchell, (1973) Soil Biol. Biochem. Vol. 5, 721-728; Papavizas, (1973) Soil Biol. Biochem, Vol. 5, 709-720.
Kawamoto and Lorbeer (Plant Dis. Reptr., Vol. 60, 189-191, 1976) report that onion seedlings were protected from damping off, caused by a particular strain of Fusarium oxysporum by infesting the onion seedlings with Pseudomonas cepacia Burkh strain 64-22. This P. cepacia strain (64-22) was reported to be recovered from the root, root-stem zone and seed coat on 18-day old seedlings from inoculated seeds. Live cells of P. cepacia, 64-22 were reported to inhibit Fusarium oxysporum f. sp. cepae, while dead cells and culture filtrate did not. The authors stated that the mechanism through which P. cepacia protects young seedlings was open to speculation. The authors concluded that the experiments reported at least supported the feasibility of biological control measures to improve onion seedling stand, but that "at present we could not recommend infesting onion seed with P. cepacia for commercial plantings . . . ", presumably because some strains of P. cepacia have been reputed to be pathogenic to onions.
R. D. Lumsden (Phytopathology Vol. 72, 709, 1982) reported that a strain of P. cepacia is antagonistic to Pythium aphanidermatum and protects cucumber seedlings from infection by this fungus in soil. In U.S. Pat. No. 4,588,584, to Lumsden et al, a new biotype of P. cepacia designated SDL-POP-S-1 is described as protecting cucumber and peas from Pythium disease. Protection is afforded through bacterial inoculation of seeds.
Another strain of Pseudomonas cepacia protected Chinese Aster against wilt caused by Fusarium oxysporum f. sp. callistephi in greenhouse and field tests (Cavileer et al, American Phytopathological Society Annual Meeting, Abstract No. 522, 1985).
One isolate of Enterobacter cloacae, E. cloacae EcET-501, in addition to its ability to fix nitrogen, has been shown to have biocontrol potential for Fusarium wilt of cucumber (Sneh et al, Phytopathology, Vol. 74, 115-1124, 1984), Phythium blight of turfgrass (Nelson et al, Phytopathology, Vol. 82, 206-210, 1992), Sclerotinia dollar spot of turf (Nelson et al, Plant Dis., Vol. 75, 510-514, 1991), Pythium seed rot of vegetables (Chao et al, Phytopathology, Vol. 76, 60-65, 1986; Hadar et al, Phytopathology, Vol. 73, 1322-1325, 1983; Nelson et al, Phytopathology, Vol. 76, 327-335, 1986), and some postharvest diseases of fruit (Wilson et al, Phytopathology, Vol. 77, 303-305, 1987; Wilson et al, Plant Dis., Vol. 69, 375-378, 1985). In addition to Phythium species, this E. cloacae is also effective in suppressing species of Fusarium and Rhizopus in the soil (Wilson et al, Phytopathology, Vol. 77, 303-305, 1987; Campbell, Biological Control of Microbial Plant Pathogens, New York: Cambridge University Press, 1989).
U.S. Pat. No. 4,588,584 (Lumsden et al, 1986) discloses that a variety of bacterial strains including members of the genera Pseudomonas, Bacillus, and Enterobacter were found to be antagonists of Fusarium. Among the fungal antagonists, root-colonizing Pseudomonads, P. fluorescens and nonfluorescent P. cepacia were the most numerous. The patent also discloses that Enterobacter cloacae was a medium to poor colonizer.
While various biocontrol agents for control of pathogenic fungi are known in the art, there still remains a need for an effective biocontrol agent for pathogenic fungi, especially Fusarium, and especially a biocontrol agent which is a symbiotic endophyte. The present invention described below is an endophytic symbiont and is antagonistic to pathogenic fungi. The present invention provides a method for controlling pathogenic fungi in agricultural crops which is different from the prior art biocontrol agents.